In a small skillet, saute onion in 1 tablespoon oil until crisp-tender. Add garlic; cook 1 minute longer.
Transfer to a 5-qt. slow cooker. Stir in ¾ cup broth, olives, tomatoes, tapioca, lemon zest and oregano. Add chicken. Cover and cook on low for 3-½-4 hours or until chicken is tender. If desired, cut chicken into pieces.
In a large saucepan, bring remaining oil and broth to a boil. Stir in couscous. Cover and remove from the heat; let stand for 5 minutes or until broth is absorbed. Serve with chicken; sprinkle with feta cheese.
